POSIX: Add code for shuffling file descriptors.

When forking a child process, one often wants to move existing file
descriptors to well known locations (stdout, stderr etc). However,
this is a process bedeviled with corner cases. Consider the case where
you open two file descriptors, get assigned fds 1 and 0 for them and
wish to shuffle them so that they end up in slots 0 and 1. Our current
code fails in this case.

We also have a problem where we're currently trying to mark file
descriptors as close-on-exec rather than closing them in the child
process. This is inherently broken in a multithreaded process where
another thread can open a file descriptor and race the loop which is
trying to mark them.

Thus, on Linux we switch to close-after-fork where we known that no
other threads exist in the process. On Mac, the code is sufficiently
different that this simple fix isn't applicable and one of the Mac
folks will need to take a look.

http://codereview.chromium.org/100127
BUG=11174


git-svn-id: svn://svn.chromium.org/chrome/trunk/src@14978 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 92 insertions, 0 deletions

diff --git a/base/file_descriptor_shuffle.cc b/base/file_descriptor_shuffle.cc
new file mode 100644
index 0000000..1426155
--- /dev/null
+++ b/base/file_descriptor_shuffle.cc
@@ -0,0 +1,92 @@
+// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/file_descriptor_shuffle.h"
+
+#include <errno.h>
+#include <unistd.h>
+
+#include "base/logging.h"
+
+namespace base {
+
+bool PerformInjectiveMultimap(const InjectiveMultimap& m_in,
+                              InjectionDelegate* delegate) {
+  InjectiveMultimap m(m_in);
+  std::vector<int> extra_fds;
+
+  for (InjectiveMultimap::iterator i = m.begin(); i != m.end(); ++i) {
+    int temp_fd = -1;
+
+    // We DCHECK the injectiveness of the mapping.
+    for (InjectiveMultimap::iterator j = i + 1; j != m.end(); ++j)
+      DCHECK(i->dest != j->dest);
+
+    const bool is_identity = i->source == i->dest;
+
+    for (InjectiveMultimap::iterator j = i + 1; j != m.end(); ++j) {
+      if (!is_identity && i->dest == j->source) {
+        if (temp_fd == -1) {
+          if (!delegate->Duplicate(&temp_fd, i->dest))
+            return false;
+          extra_fds.push_back(temp_fd);
+        }
+
+        j->source = temp_fd;
+        j->close = false;
+      }
+
+      if (i->close && i->source == j->dest)
+        i->close = false;
+
+      if (i->close && i->source == j->source) {
+        i->close = false;
+        j->close = true;
+      }
+    }
+
+    if (!is_identity) {
+      if (!delegate->Move(i->source, i->dest))
+        return false;
+    }
+
+    if (!is_identity && i->close)
+      delegate->Close(i->source);
+  }
+
+  for (std::vector<int>::const_iterator
+       i = extra_fds.begin(); i != extra_fds.end(); ++i) {
+    delegate->Close(*i);
+  }
+
+  return true;
+}
+
+bool FileDescriptorTableInjection::Duplicate(int* result, int fd) {
+  do {
+    *result = dup(fd);
+  } while(*result == -1 && errno == EINTR);
+
+  return *result >= 0;
+}
+
+bool FileDescriptorTableInjection::Move(int src, int dest) {
+  int result;
+
+  do {
+    result = dup2(src, dest);
+  } while (result == -1 && errno == EINTR);
+
+  return result != -1;
+}
+
+void FileDescriptorTableInjection::Close(int fd) {
+  int result;
+
+  do {
+    result = close(fd);
+  } while (result == -1 && errno == EINTR);
+}
+
+}  // namespace base